The regulation of development and cell proliferation in higher organisms involves signaling through receptor tyrosine kinases (RTK). Ligand binding to the extracellular domain of RTKs induces receptor dimerization or oligomerization and stimulates their intrinsic tyrosine kinase activity (Honegger et al. (1990); Kashles et al. (1991); Ueno et al. (1991); Yarden and Schlessinger (1987); Yarden and Schlessinger (1987a)). As a consequence, RTKs undergo autophosphorylation, causing further changes in receptor configuration and providing specific docking sites for cytoplasmic signaling proteins containing Src-homology 2 (SH2) or phosphotyrosine binding (PTB) domains (Kavanaugh et al. (1995); Koch et al. (1991); Songyang et al. (1993)).
RTKs are divided into families on the basis of their structural organization (van der Geer et al. (1994)), Eph receptors forming the largest known family, with at least 14 members (Pasquale (1997); Zhou (1998); Zisch and Pasquale (1997)). Ephs bind a group of ligands known as ephrins (Eph family receptor interacting), eight of which are currently known, all membrane anchored either by glycosylphosphatidylinositol (GPI) (ephrinA1-A5), or a trans-membrane domain (ephrinB1-B3) (Drescher (1997); Pasquale (1997)). Eph receptors are divided into two groups based upon their ligand binding characteristics, EphA or EphB, according to the class of ephrin bound (Brambilla et al. (1995); Ciossek and Ullrich (1997); Gale et al. (1996); Kozlosky et al. (1995); Park and Sanchez (1997)); although receptor-ligand specificity is degenerate within a group (Zhou (1998)). It is a characteristic of the Eph receptor family that their ligands must be membrane bound in order to be active (Davis et al. (1994); Sakano et al. (1996); Winslow et al. (1995)). This absolute requirement for membrane anchorage of the ligand makes the formation of cell-cell contact an obligatory event in activation of the Eph receptors. Consequently, activated receptors are concentrated in areas of cell-cell contact.
The Eph receptors and their ligands are typically most highly expressed in neural and endothelial cells (Zhou (1998)) and most descriptions of their function concern the development of the nervous system and angiogenesis (Drescher et al. (1995); Friedman et al. (1996); Hornberger et al. (1999); Gao et al. (1999); Ciossek et al. (1998); Daniel et al. (1996); O'Leary et al. (1999); Pandey et al. (1995); Adams et al. (1999); Wang. et al. (1998); Yue et al. (1999)). Upon the formation of cell-cell contact, Eph receptor signaling results in reorganization of the actin cytoskeleton and integrin activation (Becker et al. (2000); Miao et al. (2000); Zou et al. (1999); Holland et al. (1997); Huynh-Do et al. (1999)). As a result, Eph receptors generate adhesive or repulsive signals and in the neural system can guide the movement of axonal growth cones, cell migration and synapse formation (Drescher et al. (1995); Hornberger et al. (1999); Ciossek et al. (1998); Yue et al. (1999); Bohme et al. (1996); Flanagan et al. (1998); Hsueh et al. (1998); Krull et al. (1997); Monschau et al. (1997); Nakamoto et al. (1996); Mellitzer et al. (1999); Smith et al. (1997); Xu et al. (1999); Torres et al. (1998)).
The most recently identified member of the Eph family is the orphan EphB6 receptor, with a structure typical of the EphB subfamily (Gurniak et al. (1996); Matsuoka et al. (1997)). While structural analysis of EphB6 reveals conservation of the major EphB receptor autophosphorylation sites (Y638 and Y644), there are several critical alterations in the tyrosine kinase domain. These include substitution of a crucial lysine residue in the ATP binding site, resulting in a receptor that does not demonstrate detectable kinase activity (Gurniak et al. (1996); Matsuoka et al. (1997)). This casts doubt upon the ability of EphB6 to undergo tyrosine phosphorylation upon ligand stimulation and thus to initiate signaling cascades in the cytoplasm. However, analogy with ErbB-3, a well-characterized catalytically inactive member of the EGF receptor family, suggests that EphB6 may form hetero-oligiomers with catalytically active family members. And similarly, as a result of trans-phosphorylation by these active receptors, EphB6 may recruit cytoplasmic signal transducing molecules.
Unlike other receptor tyrosine kinases, EphB6 is predominantly expressed in the thymus (Gurniak et al. (1996)), suggesting that it may play an important role in T cell differentiation. Current evidence suggest that Eph receptors may directly interact with the TCR (T cell receptor) signaling pathway. Eph receptors can regulate integrin activation and cytoskeletal rearrangement (Becker et al. (2000); Miao et al. (2000); Zou et al. (1999); Holland et al. (1997); Huynh-Do et al. (1999)), both crucial events in TCR induced responses (Holsinger et al. (1998); Abraham et al. (1999); Bleijs et al. (1999); (Ticchioni et al. (1993); Valitutti et al. (1995); Wulfing et al. (1998); Wulfing et al. (1998); Snapper et al. (1998); Viola et al. (1999); Vivinus-Nebot et al. (1999)). Moreover, several Eph receptors also bind the T cell kinase Fyn (Choi et al. (1999); Ellis et al. (1996)). Indeed, high levels of EphB6 expression have been detected in a population of human peripheral T lymphocytes, but not in B cells (Shimoyama et al. (2000)). Despite its lack of kinase activity, ephrin-B1-stimulated EphB6 undergoes typrosine phosphorylation, which is provided by a catalytically active member of the EphB subfamily. This initiates its downstream signaling. The Jun N-terminal kinase (JNK) cascade (Becker et al. (2000)) is the major pathway downstream of the Eph receptor family, and is one of the key regulators of T cell apoptosis (Sabapathy et al. (1999); Baker et al. (1998)). It is currently not clear whether the Eph receptor family or any members, including the EphB6 receptor, have a role in such apoptosis. Regulation of this aspect of the immune system continues to be desirable.